The invention relates to an electrophotographic copying process, and more particularly, to an electrophotographic copying process which permits either a positive or a negative copy image, as considered with respect to the image of an original, to be selectively obtained.
A variety of electrophotographic copying processes have been proposed in the prior art which permit a positive or a negative copy image, as considered with respect to the image of an original, to be selectively and arbitrarily obtained. One technique utilizes a photosensitive member 1 for electrophotography as shown in FIG. 1 which includes a conductive layer 2 on which a photoconductive layer 3 is laminated. FIG. 2(I) illustrates a procedure followed when a positive copy image of an original is desired. As shown, the photosensitive member is initially charged to the negative polarity in a uniform manner and is then subjected to an imagewise irradiation to form an electrostatic latent image. A toner 4 which is charged to the positive polarity is deposited on the latent image for developing purpose, and the toner image is transferred onto a record sheet to provide a positive copy image. On the other hand, FIG. 2(II) illustrates a procedure followed when a negative copy image is desired. Initially the photosensitive member is uniformly charged to the positive polarity, in contradistinction to the initial charging to the negative polarity as illustrated in FIG. 2(I), and is then subjected to an imagewise exposure to form an electrostatic latent image. Again a toner 4 which is charged to the positive polarity is deposited on the latent image to produce a toner image, which is then transferred onto a record sheet to produce a negative copy image. To enable these procedures, it is essential that the photoconductive layer 3 exhibits substantially uniform charge retention and light sensitivity when it is charged to either the positive or the negative polarity. Zinc oxide is known as a suitable material to form the photoconductive layer 3 which satisfies such requirement.
However, it is to be noted that the above requirement is not satisfied by a number of photosensitive materials including Se, Se alloys, PVK (polyvinyl carbazole) containing sensitizer or the like which are frequently used in an electrophotographic system of toner image transfer type. Accordingly, the choice of material which forms the photoconductive layer 3 is greatly limited when the above procedures are to be adopted. In addition, when the above procedure illustrated in FIG. 2(II) is employed to produce a negative copy image, the toner 4 which is charged to the positive polarity is deposited on a region corresponding to the bright areas of a light image where no charge of the latent image is present, and hence it exhibits a reduced adherence, thus causing an inconvenience that a negative image of a satisfactory optical density cannot be obtained.
Another electrophotographic copying process which selectively produces a positive and a negative copy image is illustrated in FIG. 3 where a photosensitive material 5 for electrophotography is employed which comprises a conductive layer 6 carrying a successive lamination of a photoconductive layer 7 and another photoconductive layer 8 which is sensitive to the ultraviolet region of the spectrum. When it is desired to obtain a positive copy image of an original, a source of radiation 9 which supplies a radiation including ultraviolet ray is utilized to illuminate an original 10, as shown in FIG. 4(I), and the light image of the original is projected through a projection lens 11 onto the photosensitive member 5 while simultaneously utilizing a corona charger 12 to charge the photosensitive member 5 to the negative polarity, for example, thus forming an electrostatic latent image. A toner which is charged to the positive polarity is deposited principally on the dark areas of the light image to form a toner image, which is then transferred onto a record sheet to produce a positive copy image. When it is desired to produce a negative copy image, an ultraviolet cut-off filter 13 is interposed between the source 9 and the original 10, as shown in FIG. 4(II), thus allowing the original 10 to be illuminated by visible light. The resulting light image is projected onto the photosensitive member 5 through the projection lens 11 while sinultaneously charging the photosensitive member to the same polarity as used during the formation of the positive image by means of the corona charger 12, thus forming an electrostatic latent image. A toner which is charged to the positive polarity is deposited principally on the bright areas of the light image to form a toner image, which is then transferred onto a record sheet to produce a negative copy image.
In the process described immediately above, the toner is deposited on those areas of the light image where the charge of the latent image is present when either a positive or a negative image is to be produced, and hence the resulting toner image has an increased magnitude of adherence, permitting a copy image of a relatively high optical density to be obtained. However, with this process, the positive or the negative copy image is selectively produced by the use of either radiation containing ultraviolet ray or visible light, and this causes inconveniences as mentioned below.
Specifically, the distribution of radiation from a usual light source contains little or no emission of ultraviolet ray. In addition, a projection lens generally exhibits a reduced transmissivity to the ultraviolet ray. The combination of these facts makes it difficult to achieve a selective projection of radiation including ultraviolet ray in one instance and visible light in another by utilizing the same light source and the same projection lens. Furthermore, with this process, there is a high residual potential in the non-image region, namely, in the bright areas of the light image where the positive image is to be obtained as illustrated in FIG. 4(I), or in the dark areas of the light image where the negative image is to be obtained as illustrated in FIG. 4(II), resulting in an image which is highly influenced by fogging. This is because an electrostatic latent image having a high contrast cannot be formed in either instance because of the incapability of providing a photoconductive layer 7 which satisfies the both requirements for producing the positive and the negative image. More specifically, when a positive image is to be produced by a procedure as illustrated in FIG. 4(I), a sufficiently high dark resistance and a high sensitivity are required for the both photoconductive layers 7 and 8. By contrast, when a negative image is to be produced by a procedure illustrated in FIG. 4(II), a high sensitivity and a reduced dark resistance is required of the photoconductive layer 7, and this requirement is opposite from the requirement imposed upon the photoconductive layer 7 when producing a positive image.